Paul Walker -- best known for his role in "The Fast and the Furious" movies -- died Saturday afternoon after a single-car accident and explosion in Southern California ... TMZ has learned.

The accident happened in Santa Clarita -- north of Los Angeles -- and according to multiple sources connected to Paul ... the actor was in a Porsche when the driver somehow lost control and slammed into a post or a tree ... and then the car burst into flames.

Several of our sources -- some who are still at the scene of the accident -- tell us Paul and another person in the car were killed. Law enforcement is still on the scene, and we're told the L.A. County Coroner's Office is on the way.

1130_paul_walker_accident_scene_twitterAt this point, it's unclear what caused the accident, or who was behind the wheel when it happened.

Paul was 40 years old.

Sources close to Paul tell us he was in Santa Clarita for a car show to support the Philippines typhoon relief effort, and had been taking friends out for rides in his new Porsche GT. The accident happened during one of those test spins.

I used to think they were terrible but I'd kind of gave it a 2nd chance with 5, and liked it a lot as well as 6, and went back and watched the others.

I also caught his other car movie he financed and directed set in South Africa about being setup by a corrupt political organization and some cops, and thought it was pretty good - especially for working on a limited budget and all. The title escapes me at the moment.

What a shame to hear, seemed like a good guy and you definitely don't wish that on anyone.

I'm just saying there's all sorts of shit i could be doing! I could watch a movie, play a video game, rub one out, read a book, learn a foreign language, etc.. But i'm still watching the assholes because i love them, and I hate myself